1. Field of the Invention
The present invention relates to an Ethernet passive optical network (EPON), and more particularly to an OAM (Operations, Administration and Maintenance) capability discovery method that may be implemented in a passive optical network.
2. Description of the Related Art
The standardization of Gigabit Ethernet and MAC technology for the ATM-PON (Asynchronous Transfer Mode Passive Optical Network) has been completed. In the ATM-PON, upward or downward transmission of frames, each of which consists of a predetermined number of ATM cells, is used for communication purposes. In particular, an OLT (Optical Line Termination) inserts downward cells in the transmission frame, and then the downward cells are distributed to each ONU (Optical Network Unit) in the PON having a tree-shaped structure. Other details of the Gigabit Ethernet and the ATM-PON protocols are described in the IEEE 802.3z dated Jan. 23, 1997. and ITU-T G.983.1 dated Oct. 13, 1998, from which the contents of both documents are incorporated herein by reference.
FIG. 1 is a block diagram showing the physical network structure of a general passive optical network (PON). As shown, the PON includes an OLT 100 and a plurality of ONUs 110-1 to 110-3 connected to the OLT 100. FIG. 1 is an example in which three ONUs 110-1 to 110-3 are connected to one OLT 100 in the Ethernet Passive Optical Network, which transmits 802.3 Ethernet frames via a point to multi-point network. Here, multiple users 120-1 to 120-3 (users or network equipments) may be connected to the ONUs 110-1 to 110-3, respectively, so that data 131-1 to 133-1 transmitted by the users 120-1 to 120-3 can be transmitted to the OLT 100 via the ONUs 110-1 to 110-3.
As shown in FIG. 1, in the case of upward transmission, all data of the ONUs are accessed by means of a TDM (Time Division Multiplexing) method. An ODN (Optical Distribution Network), which is a passive element, enables the data not to be collided with each other by means of a ranging method. In the case of downward transmission, each of the ONUs 110-1 to 110-3 receives data broadcasted by the OLT 100. Each frame in upward or downward transmission has a field arranged in a dedicated ATM cell or a general ATM cell, by which messages can be sent or received at predetermined intervals.
However, with the development of the Internet technology, more bandwidth at the subscriber's ends are required, and to address this need, a point-to-point transmission by means of Gigabit Ethernet, which is relatively low-priced and can secure a higher bandwidth in comparison to the ATM technology, is utilized. In this regard, the ATM has drawbacks in that it requires relatively expensive equipment, has limited bandwidth, and requires the segmentation of IP packets. As such, even in the PON structure of the subscriber network, the Ethernet type is more favored rather than the ATM technique.
In order to provide services by means of Ethernet-based passive optical network, it is necessary for the provision of OAM capability, which does not exist in the existing Ethernet. The OAM has been defined recently in an EPON standardization but still being developed in IEEE 802.3ah EFM TF, Draft v1.0. The OAM capability defined in the Draft v1.0 up to now is a basic function, which supports Remote Failure Indicating function, Remote Loop-back function, and Link Monitoring function, as described in IEEE 802.3ah EFM. However, since detailed operation processes for the OAM discovery method are not defined in the Draft v1.0, some problems may occur in attempts to perform the OAM protocols effectively. In other words, no uniform discovery process for the OAM capability is available between the OLT 100 and the ONU 110. As a result, any OAM capabilities defined and used by different vendors will not cooperate with each other as the frames may be unrecognizable to each other.